The present invention relates to a control arrangement concerning the air/fuel ratio in a carburetor of a combustion engine
The ratio of the air/fuel mixture (designation A/F) as far as combustion engines are concerned has lately become a more interesting matter since--for environmental reasons--engines as "clean" as possible are desired.
In order to obtain clean exhaust gases from the engine it is necessary to burn the air/fuel mixture as efficiently as possible. It has since long been a difficult problem to control the mixture ratio with regard to different operating conditions of the engine. A simple membrane carburetor of the type used for example, in chain saws has an uneven ratio of A/F mixture along the r.p.m. range (r/s) of the engine. Sample values taken on an engine with a carburetor of that kind show (FIG. 1) that the diffusion of the function A/F with regard to r/s is considerable. In FIG. 1 the A/F values are indicated in proportion (0.94, 0.96, 0.98 etc.) to an ideal value of set at a value of 1. The values have been measured on a carburtor set on a rich mixture and, therefore, the curve does not in any point reach the ideal value 1. Different settings of the adjustment screws on the carburetor indicate, however, that the uneveness of A/F cannot be eliminated, so it is necessary to apply another kind of adjustment depending on r.p.m. and load.
Due to the present invention the problem of equalizing the A/F function and setting it on an ideal value has been solved by means of a control device on the carburetor which, depending on load and r.p.m., controls the differential pressure (.DELTA.p) in the carburetor. This pressure is defined in the simplest way by considering a float carburetor (FIG. 2) where, on one hand there, is an atmospheric pressure in the chamber, and, on the other, a subpressure in the venturi tube through the carburetor. The differential pressure .DELTA.p is the difference between those two pressures. In analogy thereof a membrane carburetor has also a .DELTA.p, being the difference between the pressure prevailing on the dry side of the membrane (reference pressure) and the said subpressure. The arrangement acts on a carburetor which, in its starting position with reference pressure=atmospheric pressure, is set on a rich mixture (curve according to FIG. 1) but which in different operating conditions, due to the action of the arrangement, gets a reduced reference pressure, i.e. a subpressure, on the membrane or in the float chamber so that A/F increases (is raised) to the desired ideal value. This occurs by control of .DELTA.p. A carburetor arrangement apt to realize the said improvement on the conditions given shall, according to the invention, have the characteristics stated in claim 1.
An embodiment of the carburetor arrangement according to the invention will now be described with reference to the attached drawings.